The present invention is directed to light gauging techniques for locating the leading edge of part such as turbine blade having a complex part shape. Accurately identifying the contour of the leading edge is important because many performance characteristics of the blade are determined by the shape of its leading edge. However, because of the complexity of the blades contour obtaining accurate measurements can be both difficult and time consuming. For example, it possible to hard gauge the contour of the blade using touch probes and other contact instruments. This, however, is time consuming and adds substantial cost to a production line process.
An alternative testing method involves light gauging techniques. For example, it is possible to measure the shape of an object by mounting it on a fixture and illuminating it with a structured laser light. That is, a laser light is directed at the surface of the object and a pattern of light stripes are produced which extend across the surface of the object generally parallel to each other. The path of the stripes varies in accordance with the shape features of the part. By rotating the part, a series of images can be obtained and processed using appropriate algorithms employing known triangulation techniques. Processing of the images allows the points to be located in three dimensional (3D) space.
If the part has relatively flat contours, or if curvatures in the contours are not very pronounced, the above procedure works well. However, for an airfoil such as a turbine blade, the shape of its leading edge is relatively complex and it has been found that it is difficult to accurately define the leading edge shape using the above described technique. A technique known as “space carving” has heretofore been used to create realistic 3D models from real objects. See, for example, Automatic 3D Model Construction for Turn-Table Sequences, Fitzgibbon et al., Robotics Research Group, Department of Engineering Science, University of Oxford (publication date). While the previous usage of space carving techniques has been the replication of whole objects, the present invention is directed not at replicating the overall shape of a turbine blade, rather at accurately determining only the shape of the leading edge so-that accurate comparisons between the blade's contour and a set of acceptance criteria for the part.